Documentation on Materials and Textures
First, I think the answer to your question is "Why no documentation?" is simple. It is free open source software and the developers didn't yet have to time to do it in more details. However, if you have some knowledge in the subject you can contribute: 

https://www.blender.org/manual/about/contribute/index.html

If everyone will do so, the documentation soon will be very extensive.

Second, it is true, that the documentation on this topics is not detailed enough. I also didn't necessarily have problems with particular materials, but wanted first to know what materials I can create, how I should do it and what different parameters mean. I found many answers in the article below. It has good explanations and many pictures. Hope it will be useful for you too.

http://www.blenderguru.com/articles/cycles-shader-encyclopedia/